This concrete slab calculator tells you how much concrete to buy for a patio, shed base, walkway or equipment pad — in cubic yards for a ready-mix truck and in 80, 60 or 40 lb bags for mixing yourself — and what it will cost. Enter the slab size and thickness, adjust the waste allowance, and type in your local prices; the example prices filled in are placeholders only, so replace them with a real quote.
Bag yields come from the manufacturer: Quikrete states that an 80 lb bag of concrete mix yields about 0.60 cubic feet, a 60 lb bag about 0.45 ft³ and a 40 lb bag about 0.30 ft³. Most slabs sit on a compacted gravel base — size it with the gravel calculator — and if you are tiling the finished slab, the tile calculator handles the next step.
How to use the concrete slab calculator
- Enter the slab length and width. For a round pad, choose Circle and enter the diameter.
- Enter the thickness. 4 inches is standard for patios and walkways; 5–6 inches is common for driveways and slabs that carry vehicles.
- Keep 5–10% waste for spillage, uneven subgrade and form bowing. The default is 10%.
- Enter your ready-mix price per cubic yard and/or price per 80 lb bag.
- Compare the bulk cost against the bag cost. For larger pours, also ask about short-load fees, which some suppliers charge on small truck orders.
Formula
Bag yields used: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³.
Worked example
A 10 × 10 ft patio slab, 4 inches thick.
- Volume: 10 × 10 × (4 ÷ 12) = 33.3 ft³ = 1.23 yd³.
- With 10% waste: 1.23 × 1.10 = 1.36 yd³ (36.7 ft³).
- 80 lb bags: 36.7 ÷ 0.60 = 61.1 → 62 bags (about 2.5 tons of bags to lift).
- Cost at the example prices: 1.36 × $165 = $224 ready-mix, or 62 × $6.50 = $403 in bags.
For anything above about one cubic yard, ready-mix or a rented mixer is usually worth pricing out — mixing sixty-plus bags by hand is a long day.
Concrete needed per 100 ft² of slab
| Thickness | Cubic yards (no waste) | 80 lb bags (no waste) |
|---|---|---|
| 3 in | 0.93 | 42 |
| 4 in | 1.23 | 56 |
| 5 in | 1.54 | 70 |
| 6 in | 1.85 | 84 |
Caveats and assumptions
- Thickness and reinforcement for driveways, footings and structural slabs depend on soil, loads, climate and local code. Follow your building department and the Portland Cement Association's guidance, and consult an engineer for structural work.
- Subgrade is never perfectly flat; low spots eat concrete quickly. Measure the form depth in several places.
- Prices pre-filled in the cost fields are examples, not market averages. Ready-mix quotes often add delivery, short-load and fuel charges.
- Normal-weight concrete weighs roughly 145–150 lb per cubic foot, so even a small slab is heavy to transport and place.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
About 45. One cubic yard is 27 ft³, and an 80 lb bag yields roughly 0.60 ft³, so 27 ÷ 0.60 = 45 bags.
How much concrete do I need for a 10×10 slab?
At 4 inches thick, a 10 × 10 ft slab needs 1.23 cubic yards, or about 1.36 yd³ (62 eighty-pound bags) with 10% waste.
How thick should a concrete patio be?
Four inches is the usual minimum for patios and walkways. Driveways and slabs for vehicles are commonly 5–6 inches; check local code.
Is it cheaper to use bags or ready-mix?
Bags are usually cheaper for very small jobs under about half a yard. Above roughly one yard, ready-mix is typically cheaper and far less labour, though small loads can carry extra fees.
How much does a yard of concrete weigh?
Roughly 4,000 lb (about 2 tons), based on about 145–150 lb per cubic foot.
Sources
- Quikrete — Concrete Calculator and product yields — yield per 80, 60 and 40 lb bag
- Portland Cement Association — concrete design and construction guidance
- NIST — Unit Conversion
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.